John Lighton Synge was arguably the greatest Irish mathematician and theoretical physicist since Sir William Rowan Hamilton (1805-1865). He was a proliﬁc researcher of great originality and versatility, and a writer of great lucidity and striking ‘clarity of expression’. He made outstanding contributions to a vast range of subjects and particularly to Einstein’s theory of relativity. His approach to relativity, and to theoretical physics in general, is characterized by his extraordinary geometrical insight. In addition to bringing clarity and new insights to relativity, his geometrical approach inﬂuenced profoundly the development of the subject from the early 1960s.